Process and device for coating silicon carbide fibers

ABSTRACT

In a process for coating silicon carbide fibers with a titanium-based alloy by plasma spraying, the titanium-based alloy is sprayed onto the silicon carbide fibers by way of an electrode-free high-pressure plasma. The high-pressure plasma is generated in a microwave- or radiowave-transparent working tube with a gas inlet opening and a gas outlet opening by ignition of a process gas which has been introduced into the working tube at a pressure p≧1 bar. The plasma is maintained by absorption of microwaves or radiowaves and is passed into the working space as a plasma jet through a nozzle arranged at the gas outlet opening of the working tube.

[0001] This application claims the priority of German application 101 40 465.4, filed Aug. 17, 2001, the disclosure of which is expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] The present invention relates to a process for coating silicon carbide fibers with a titanium-based alloy by plasma spraying in which the titanium-based alloy is sprayed onto the silicon carbide fibers by way of an electrode-free high-pressure plasma. The process includes generating the high-pressure plasma in a microwave- or radiowave-transparent working tube with a gas inlet opening and a gas outlet opening by ignition of a process gas which has been introduced into the working tube at a pressure p≧1 bar, maintaining the plasma by absorption of microwaves or radiowaves, and passing the plasma into the working space as a plasma jet through a nozzle arranged at the gas outlet opening of the working tube. The invention also relates to a device for carrying out such a process which includes at least two high-pressure plasma torches, each of the high-pressure plasma torches producing one plasma jet without using electrodes. The plasma torches are arranged symmetrically with respect to one another in such a manner that the plasma jets meet at least at one point through which the silicon carbide fibers which are to be coated are guided.

[0003] It is known from European publication EP 0 615 966 B1 that PVD (physical vapor deposition) processes or sputtering processes are used to coat silicon carbide fibers with a metal matrix.

[0004] A process for forming fiber-reinforced metal matrix elements is known from European publication EP 0 358 799 B1. In this process, a silicon carbide fiber is coated with a titanium-based metal in solid form. In this known process, the coating takes place by way of a low-pressure high-frequency plasma spraying. The titanium-based metal solid is added to the process gas in powder form.

[0005] A drawback of this process is the ineffective and expensive production of the coatings. Moreover, the known plasma spraying process in the low-pressure range does not allow coating of the silicon carbide fibers in a production line.

[0006] It is an object of the invention to provide a novel process which makes it possible to provide a simple and effective coating of silicon carbide fibers in a production line. A further object of the invention is that of providing a device for carrying out the process.

[0007] These objects are achieved in accordance with the invention. Advantageous embodiments of the invention form the subject matter of dependent claims.

[0008] According to the invention, the silicon carbide fibers are coated with a titanium-based alloy by plasma spraying. The titanium-based alloy is sprayed onto the silicon carbide fibers by way of an electrode-free high-pressure plasma. The high-pressure plasma is ignited in a microwave- or radiowave-transparent working tube with a gas inlet opening and a gas outlet opening by way of a process gas which has been introduced into the working tube at a pressure p≧1 bar. The high-pressure plasma is maintained by absorption of microwaves or radiowaves. Moreover, the high-pressure plasma is introduced into the working space as a plasma jet by a nozzle arranged at the gas outlet opening of the working tube.

[0009] One advantage is effective and rapid coating of the silicon carbide fibers. Unlike in the prior art, in the process according to the invention, there is no need to generate a vacuum. This results in further advantages with regard to improved productivity and efficiency of the process according to the invention, and consequently the process according to the invention is suitable for use in a production line.

[0010] In an advantageous embodiment of the invention, the process gas for generating the high-pressure plasma according to the invention contains hydrogen. In particular, the process gas is a mixture of hydrogen and an inert gas, e.g. argon.

[0011] In a further advantageous embodiment of the invention, the titanium-based alloy can be fed to the process gas in the form of a liquid and/or solid precursor. The liquid precursor used may be titanium tetrachloride (TiCl₄) and the solid precursor used may be a titanium-based alloy powder.

[0012] If TiCl₄ is used as liquid precursor in the process gas, the following reactions occur in the plasma:

TiCl₄+2H₂→Ti+4HCl  (1)

TiCl₄+4H→Ti+4HCl  (2)

[0013] Reaction (1) is thermodynamically limited, unlike reaction (2). This means that, on account of the high temperature in a high-pressure plasma (approx. 10⁵K), the molecular hydrogen (H₂) is virtually completely dissociated. Therefore, it is predominantly reaction (2) which takes place in the high-pressure plasma, and consequently predominantly atomic hydrogen (H) is present in the plasma.

[0014] Atomic titanium is known to have a high affinity for oxygen, with the result that titanium oxide is formed. The high proportion of inert gas in the high-pressure plasma prevents formation of titanium oxide. Of course, to prevent formation of titanium oxide it is also possible for the working space additionally to be purged with an inert gas.

[0015] The process according to the invention is used to coat components for lightweight construction, in particular for fiber components made from Ti-MMC (titanium metal matrix composite) for use in aeronautical gas turbines.

[0016] In the device according to the invention, for coating silicon carbide fibers with a titanium-based alloy, at least two high-pressure plasma torches with, in each case, one plasma jet produced without the use of electrodes are arranged symmetrically with respect to one another. The arrangement is such that the plasma jets meet at least at one point, specifically at the very point at which the silicon carbide fiber which is to be coated is running. The symmetrical arrangement of the plasma torches allows homogeneous coating over the entire circumference of the fibers.

[0017] In an advantageous embodiment of the device according to the invention, three high-pressure plasma torches are arranged in such a manner with respect to one another that the plasma torches form an angle of 120° with respect to one another. This results in optimum and homogeneous coating of the fibers. Of course, it is also possible for more than three plasma torches to be arranged symmetrically with respect to one another.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

[0018] The only drawing shows an exemplary embodiment of the device according to the invention with three plasma torches.

DETAILED DESCRIPTION OF THE INVENTION

[0019] In a high-pressure plasma torch 1, an electrode-free high-pressure plasma 3 is produced in a working tube 2 and is passed into the working space 5 as plasma jet 6 by way of a nozzle 4. The high-pressure plasma (3) is generated by igniting a process gas which is passed through the gas inlet opening (8) of the working tube (2). In addition, the liquid and/or solid precursors are fed through the gas inlet opening 8 of the working tube 2 to the process gas and therefore to the high-pressure plasma 3. Particularly when solid precursor is being used, the result is optimum partial melting of the particles.

[0020] The plasma torches 1 are arranged at an angle of 120° with respect to one another, the plasma jets 6 from the various plasma torches 1 meeting at a point at which the silicon carbide fiber 7 which is to be coated is running. This ensures homogeneous coating of the silicon carbide fiber 7.

[0021] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

I claim:
 1. A process for coating silicon carbide fibers with a titanium-based alloy by plasma spraying, the titanium-based alloy being sprayed onto the silicon carbide fibers by way of an electrode-free high-pressure plasma, comprising: generating the high-pressure plasma in a microwave- or radiowave-transparent working tube with a gas inlet opening and a gas outlet opening by ignition of a process gas which has been introduced into the working tube at a pressure p≧1 bar, maintaining the plasma by absorption of microwaves or radiowaves, and passing the plasma into the working space as a plasma jet through a nozzle arranged at the gas outlet opening of the working tube.
 2. The process according to claim 1, wherein the process gas for generating the high-pressure plasma contains hydrogen.
 3. The process according to claim 1, wherein the titanium-based alloy is fed to the process gas in the form of a liquid precursor, a solid precursor, or both a liquid and solid precursor.
 4. The process according to claim 3, wherein the liquid precursor is titanium tetrachloride.
 5. The process according to claim 3, wherein the solid precursor is a titanium-based alloy powder.
 6. The process according to claim 2, wherein the process gas contains a mixture of hydrogen and an inert gas.
 7. The process according to claim 2, wherein the titanium-based alloy is fed to the process gas in the form of a liquid precursor, a solid precursor, or both a liquid and solid precursor.
 8. The process according to claim 7, wherein the liquid precursor is titanium tetrachloride.
 9. The process according to claim 7, wherein the solid precursor is a titanium-based alloy powder.
 10. A device for carrying out a process according to one of the preceding claims, comprising at least two high-pressure plasma torches, each of the high-pressure plasma torches producing one plasma jet without using electrodes, the plasma torches being arranged symmetrically with respect to one another in such a manner that the plasma jets meet at least at one point through which the silicon carbide fibers which are to be coated are guided.
 11. The device according to claim 10, wherein three of said high-pressure plasma torches are arranged in such a manner that they are at angles of 120° with respect to one another. 